TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 



133 



Section through the Foramen Ovale of the Heart (Fig. 135). The level of this sec- 

 tion is cranial to that of the previous figure and shows the septum primum interrupted dor- 

 sally to form the foramen ovale. Each atrium communicates with the ventricle of the same 

 side through the alrio-ventricular foramen. Between these openings is the endocardial cush- 

 ion, which in part froms the anlages of the tricuspid and bicuspid valves. The atria are 

 marked off externally from the ventricles by the coronary sulcus. Between the two ven- 

 tricles is the interventricular septum. The ventricular walls are thick and spongy, forming 

 a network of muscular cords, or trabecula, surrounded by blood spaces, or sinusoids. The 

 trabeculae are composed of muscle cells, which later become striated and constitute the 

 myocardium. They are surrounded by an endothelial layer, the endocardium. The mam- 

 malian heart receives all its nourishment from the blood circulating in the sinusoids until 

 later, when the coronary vessels of the heart wall are developed. The heart is surrounded 

 by a layer of mesothelium, the epicardium, which is continuous with the pericardial meso- 

 thelium lining the body wall. 



Foramen ovale 

 R. alritim 



R. atrio-ventricular foramen 



R. ventricle 



Interventricular septum 



L. atrium 



Septum I 



Endocardial cushion 



L. atrio-venlricular foramen 



L. ventricle 



FIG. 135. Transverse section through the foramen ovale of the heart in a 10 mm. pig embryo. 



X 22.5. 



Section through the Liver and Upper Limb Buds (Fig. 136). The section is marked 

 by the presence of the upper limb buds, the liver, and the bifurcation of the trachea to 

 form the primary bronchi of the lungs The limb buds are composed of dense, undiffer- 

 entiated mesenchyme, surrounded by ectoderm which is thickened at their tips. The 

 seventh pair of cervical ganglia and nerves are cut lengthwise, showing the spindle-shaped 

 ganglia with the dorsal root fibers taking origin from their cells. The ventral root fibers 

 arise from the ventral cells of the mantle layer and join the dorsal root to form the nerve 

 trunk. On the right side a short dorsal ramus supplies the anlage of the dorsal muscle 

 mass. The much larger ventral ramus unites with those of other nerves to form the brachial 

 plexus. 



The descending aorta have now fused and the seventh pair of dorsal intersegmental 

 arteries arise from the dorsal aorta. From these intersegmental arteries the subclavian 

 arteries are given off two sections caudad in the series. Lateral to the aorta are the pos- 

 terior cardinal veins. The esophagus, ventral to the aorta, shows a very small lumen, while 

 that of the trachea is large and continued into the bronchi on either side. Adjacent to 

 the esophagus are the cut vagus nerves. The lung anlages project laterally into the cres- 

 centic pleural cavities, of which the left is separated from the peritoneal cavity by the sep- 



